Unveiling mechanistic implications of CO2 reduction driven by protic ionic liquids on electrodeposited metal oxide thin films
Date
2021
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Publisher
University of Delaware
Abstract
Since the dawn of the industrial revolution, unrestrained combustion of fossil fuels to meet energy demands has led to a rise in atmospheric CO2 levels that have a potentially huge impact on global climate change. Despite being one of the key greenhouse gases associated with modern lifestyle, CO2 is also potentially a primary source for C1 carbon for the synthesis of fuels and other commodity chemicals such as carbon monoxide, formic acid, and methanol. Consequently, extensive research efforts have been directed towards developing strategies for building a sustainable energy source. Electrochemical CO2 conversion is one such strategy whereby the excess electricity from renewable sources such as solar can be stored in the chemical bonds, by using the electrons to convert CO2 into fuels and chemical precursors. Heterogenous electrocatalytic CO2 reduction is a complex process involving many interrelated phenomena including both the electrode and electrolyte compositions. Among various electrochemical setups, the combination of relatively earth abundant metals and ionic liquids electrolytes has gained attention largely due to the ability to perform CO2 reduction at moderate overpotentials using metals typically associated with poor electrocatalytic properties. ☐ In this thesis, in depth understanding of CO2 electroreduction in the presence of a protic ionic liquid, [DBU-H]PF6, is established. First, in Chapter 2, a model electrocatalyst material is developed from electrodeposited multi-metallic oxides with varying ratios of Ag and Sn to investigate how the electrode composition affects the electrocatalytic performance of such composite materials. These electrodeposited AgSnOx thin films promote electrocatalytic CO2 reduction to CO and HCOO– with fast kinetics in the presence of [DBU-H]PF6. A tunable product distribution is obtained by varying Ag concentration, whereby significant increase in CO vs. HCOO– selectivity (up to 99%) is achieved for Ag content from 25 - 75%. Increased AgOx content also promotes an anodic shift in the onset potential for CO2 compared to the monometallic films hence increased total current geometric current density up to 30 mA/cm2 at less than 800 mV overpotentials. Moreover, ex-situ XPS analysis, indicates that Ag promotes the growth and stability of oxidized SnOx surface species which are well known to promote both low onset potentials and high selectivity for eCO2RR over HER. Additionally, electrochemical impedance spectroscopy (EIS) coupled with analysis of the distribution of relaxation times (DRT) is used to probe the dynamics between the metal-oxide, IL, and CO2 at the electrode-electrolyte interface to gain insight into the pathway(s) by which the oxide species drive CO2 reduction. For instance, DRT analysis shows the presence of multiple relaxation processes related to both adsorption and diffusion-controlled processes associated with enhanced CO2 reduction activity. ☐ In Chapter 3, a model electrodeposited Ni thin films provide a new picture of the dynamic processes that govern the activity of a Ni surface during CO2 hydrogenation. Nontrivial effects of polarization induced electrolyte adsorption on selectivity and activity of the Ni electrodes in MeCN with [DBU-H]+ as proton source are demonstrated. The voltammetric and electrolysis results from various proton sources, reveal that both the cation identity and pKa of the most acidic proton source in the electrolyte are crucial for CO evolution on nickel cathodes during CO2 electroreduction whilst suppressing hydrogen production in a nonaqueous media. It is shown that cathodic polarization induces surface roughening resulting in significant formation of an anion rich SEI layer independent of either N2 or CO2 atmosphere. XPS characterization is suggestive of formation of a complex nickel hydroxy-anion species during electrochemical analysis. Finally, using [DBU-H]PF6 and 1.1 M H2O as model proton sources in CO2 saturated MeCN, it is proposed that after the formation of an anion rich SEI layer, the reactivity towards CO2 is more pronounced than any other proton source so long as the bulk pKa of the proton source is about 24. ☐ In Chapter 4, the non-destructive electro-analytical technique of voltammetry is used as a diagnostic tool to dissect the possible reaction pathways for CO2 reduction in the presence of [DBU-H]PF6 ionic liquid as an electrolyte additive. Electrodeposited Ag and Bi thin films are used as model cathodes since they individually exhibit vastly different electrocatalytic performances despite a similar voltammetric shape. Linear sweep voltammetry is used to characterize the electrocatalytic activity of silver and bismuth thin films for CO2 reduction in acetonitrile in the presence of [DBUH]+ as proton source. Semi-derivative voltammetry is used as a convolution technique to probe the effect of various parameters such as the scan rate on the prominent features of the voltammetry plots. In this chapter, it is proposed that regardless of the electrode composition, electrocatalytic CO2 reduction in MeCN in the presence of [DBU-H]PF6 occurs via a potential dependent, proton-coupled electron transfer pathway involving the formation of a hydrogen bonded complex formed by the heteroconjugation of eCO2RR co-products such as [DBU] and H2O with adsorbed intermediates such as COOH or CO2•–. A combination of the voltammetric results and observations from other works in the Rosenthal Group lead to the hypothesis that the selective enhancement of CO2 reduction over the Ag and Bi cathodes stems from the activation of CO2 via hydrogen bonding induced by the basicity of [DBU] conjugate base. These results establish the synergistic effects of organic/inorganic hybrid as a complementary method for tuning selectivity in CO2‐to‐fuels catalysis. ☐ In Chapter 5, the analysis of the distribution of relaxation times technique is used to deconvolute EIS data to identify underlying relaxation processes during CO2 electrocatalytic CO2 reduction at rotating electrode setup. In this Chapter electrodeposited Bi thin films are used as the model electrocatalyst. The DRT analysis indicates that in the frequency range of interest, there are three main regions with distinct distributions of relaxation times that can provide information about the kinetics of charge transfer under specific conditions relevant to CO2 reduction at bismuth cathodes. Three major polarization processes during CO2 electroreduction on bismuth cathodes in the presence of [DBU-H]PF6 as a proton source are identified. These are contact resistance between the electrodeposited thin film and the glassy carbon substrate, charge transfer during CO2 activation as well as mass transport to and from the electrode. The results further corroborate the results on the proposed mechanism of CO2 presented in Chapter 4 as well as explain the DRT features first presented in Chapter 2. Crucially, the DRT analysis reveals the presence of potential dependent features suggestive of a very fast chemical step prior to electron transfer at the rate determining step. This reaction is faster than natural transport since the studied are performed under RDE. These results support the conclusion from Chapter 4 that such a mechanism explains the peculiar shape of the LSVs involving [DBU-H]+ as a proton source. Tafel measurement obtained by polarization resistance from EIS agrees with Tafel data from voltammetry studies pointing towards the rate determining step involving proton transfer especially at more anodic potentials. Overall, the results so far lead to the hypothesis that CO2 reduction in the presence of [DBU-H]PF6 involves a transition state whereby a H-bonded complex is formed as intermediate during proton transfer. The extent to which such a transition state is maintained even at more cathodic potentials is dependent on the solvent polarity or simply its hydrogen donor ability. ☐ Lastly, Chapter 6, is an exploration of the electrocatalytic effect of alloying Sn, Pb and Bi, which are all reported as being individually active catalysts for CO2 reduction to different extents. It is demonstrated that the well-known soldering alloy Bi50Sn22Pb28 (Rose’s Metal = RM) promotes the conversion of CO2 to CO in MeCN electrolyte containing millimolar concentrations of the non-protic ionic liquid additive [BMIM]OTf. Planar RM electrocatalysts evolve CO with Faradaic efficiencies in excess of 80% with average geometric current densities of jtot = 3–10 mA/cm2. Ex-situ XPS analysis also showed evidence for the accumulation of metal oxides on the RM surface during CO2 electrocatalysis, which may influence the activity of the alloy.
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Keywords
CO2 electroreduction, Protic ionic liquids, Energy sources